When a moving organ is scanned by a medical image diagnosis apparatus represented by the X-ray CT apparatus, artifacts due to motion, so-called motion artifacts, are caused in the obtained tomographic image. As a method of reducing the motion artifacts, there is a method of controlling the acquisition timing of projection data or processing the acquired projection data based on electrocardiographic information or respiration information measured using a biometric sensor, such as an electrocardiograph or a respiratory sensor. With reference to the scanning of the heart as an example, there is a method in which electrocardiographic information measured using an electrocardiograph is collected together with projection data and projection data corresponding to an arbitrary cardiac phase is extracted from the collected projection data to perform image reconstruction and as a result, a tomographic image of the heart corresponding to the cardiac phase is obtained. This method is called an electrocardiographic synchronous reconstruction method.
In order to reduce the motion artifacts in the electrocardiographic synchronous reconstruction method, it is important to specify the cardiac phase corresponding to the projection data, which is used for image reconstruction, as an optimal phase, for example, as a cardiac phase in which the movement amount of the heart is smallest. PTL 1 discloses reconstructing a plurality of images of different cardiac phases, generating a plurality of difference images of different cardiac phases from the plurality of reconstructed images, and determining a specific cardiac phase based on the amount of movement corresponding to a plurality of cardiac phases calculated using the generated difference images. That is, a cardiac phase in which the sum of absolute values of the pixel values of the generated difference images is smallest is determined as a cardiac phase in which the amount of movement is small.